Resonance enhanced coherent anti-stokes raman scattering and laser induced fluorescence applied to CH radicals: a comparative study

Abstract.

Resonance enhanced CARS and LIF have been applied to the CH radicals in a microwave excited Ar/H$_2$/CH$_4$ plasma ($p = 21$ Pa, $P_{\rm Mikro} = 2.7$ kW). Both techniques yield similar nonthermal rotational population distributions of CH(X$^2\Pi_r$) in its vibrational ground state ($v=0$), which can be described by two rotational temperatures, $T_{\rm rot,1} \approx 600$ K being in the order of the gas temperature for rotational states with $N \le 7$, and a considerably higher $T_{\rm rot,2}$ for the higher rotational states. This result is in good agreement with previous resonance CARS and LIF measurements in similar plasmas. With resonance CARS additional measurements on CH in the $v=1$ state could be performed yielding a vibrational temperature of 2440 K, the total CH density was about $1.6 \times 10^{18}$ m$^{-3}$. The detection limits of both techniques are determined, in our case about $2 \times 10^8$ CH radicals per quantum state in the detection volume, and their advantages and disadvantages are discussed.